The field in question is that of power transmission and it relates more particularly, but not exclusively, to reversible drive systems for motor vehicle engines, in particular between a crankshaft and a starter-alternator that are interconnected by a belt. Such a pulley may advantageously also be fitted to other members of the drive system, in particular members that are not reversible: a compressor, a water pump, etc.
The invention may also be applied in any field that requires power to be transmitted by rotary machines or motors, e.g. in industrial installations.
It is known to integrate the starter function for an internal combustion engine with an alternator, then referred to as a starter-alternator. Such integration makes it possible to omit the starter, i.e. a heavy toothed ring coupled to a high-inertia flywheel and to the electrical starter motor.
The starter-alternator acts both as a motor for starting the engine via a flexible link and the crankshaft pulley which then acts as a brake, and also as an alternator that is driven once the engine has started and that serves in particular to recharge the battery.
Integration can be achieved either by directly coupling the alternator which is mounted on the crankshaft, referred to as an integrated starter-alternator (ISA), or else by coupling a starter-alternator via a belt transmission (as a replacement for a conventional alternator), referred to as a separate starter-alternator (SSA).
The separate configuration that uses the belt to transfer drive between the two members, i.e. the crankshaft and the alternator, enables assembly to be highly adaptable and starting to be very smooth. The belt is of the poly-V type, cog type, or trapezoidal type. The present invention relates to this separate configuration.
Unfortunately, starting an engine involves dynamic phenomena that are violent and that fluctuate rapidly as a function of internal friction varying depending on the positions of the connecting rods and on successive cylinders being put into compression. As shown by the timing diagrams of accompanying FIG. 16, during this high stress starting stage, the crankshaft speed VB increases suddenly (at an instant t0 at a few seconds) and the crankshaft torque changes from being driven to driving. Under such conditions, the starter-alternator torque CAD likewise fluctuates, respectively from driving (positive peaks Cm) to being driven (negative peaks Cr).
After starting, once running, the crankshaft drives and the starter-alternator is driven. In operation, the instantaneous speed of the crankshaft then fluctuates in substantially sinusoidal manner: this phenomenon is referred to as engine “acyclism”. These fluctuations in speed are transmitted by the belt to driven members such as the alternator, compressor, water pump, etc.
Since inertia in the starter-alternator is high, the dynamic torque generated thereby is of large amplitude, and alternates between being positive and negative in spite of rotation taking place in one direction only: this torque leads to large variations in tension, with high tension levels stressing the components (belts, tensioners, winders), and with low tensions leading to poor drive (slip) and to noise.
At engine starting speeds, the driving torque that needs to be applied to the crankshaft can reach values of about 90 newton meters (N.m), and even values that are considerably greater, being as great as 150 N.m to 180 N.m. Unfortunately, a conventional decoupling pulley mounted on an alternator is not capable of delivering driving torque greater than a value that is typically about 30 N.m at most, which means that the crankshaft can be driven with torque of 90 N.m using a typical reduction ratio of 3 between the two members.
In particular, when torque needs to be delivered to the crankshaft at significantly higher values, torque variations are large and the resulting jolts cause slip and vibration in the transmission via the belt.
It should also be observed that the positive and negative torque values are not necessarily symmetrical, given asymmetries in the damping effects (due to friction) and mechanical dissymmetries in the movements, both while starting and while running.
In order to absorb dynamic variations in started mode, it is known to make use of freewheels as described in U.S. Pat. Nos. 5,676,225, 6,093,991, or 6,237,736, or to use resilient freewheels as described in U.S. Pat. No. 6,083,130 or in European patent No. 0 517 184. In principle, freewheel or resilient freewheel systems are completely incompatible with reversible use, e.g. on a pulley of a starter-alternator for driving a crankshaft, because of the change of direction in the torque.
There also exist decoupler pulleys fitted with an elastomer ring of stiffness that determines an angular deformation. They enable irregularities in speed or torque to be absorbed and they are therefore essentially dedicated to filtering at operating speeds when crankshafts are turning slowly.